1. Technical Field
The present invention relates to a light flux diameter-expanding element which expands a light flux diameter, and a display apparatus which makes light-modulated light flux be incident on the eyes of a user.
2. Related Art
In a retina scanning type display apparatus which makes light-modulated light flux be incident on the eyes of a user, a light source portion which emits light flux for displaying an image, a scanning optical system which applies the light flux emitted from the light source portion in two directions, and a light guide system which reflects the light flux applied by the scanning optical system and makes the light flux be incident on the eyes of user, are provided. In addition, in the retina scanning type display apparatus, when the light flux is small, the light flux is not incident on a pupil in a case in which a position of the pupil is changed, therefore, a defect of the image, and the like are generated. Here, a light flux diameter-expanding element (pupil expanding element) is provided in the retina scanning type display apparatus.
Meanwhile, as the light flux diameter-expanding element (pupil expanding element), a light flux diameter-expanding element has been proposed in which two diffraction gratings (first diffraction grating and second diffraction grating) face each other and diffraction angles thereof are coincident with each other by making both grating periods thereof be equal to each other (refer to JP-A-7-72422).
However, in a configuration in which the two diffraction gratings (first diffraction grating and second diffraction grating) face each other, there are problems which will be described later with reference to FIG. 10. In FIG. 10, diffraction light which is emitted in parallel to incident light flux L0 is illustrated, and emission light beams (+1-st order diffraction light, 0-th order diffraction light, and −1-st order diffraction light) emitted from a first diffraction grating 11 are emitted in parallel to each other with intervals of 3 mm therebetween as respectively −1-st order diffraction light, 0-th order diffraction light, and +1-st order diffraction light in a second diffraction grating 12, for example. For this reason, light flux L1 including the −1-st order diffraction light, the 0-th order diffraction light, and the +1-st order diffraction light emitted from the second diffraction grating 12 has a diameter of 6 mm. Accordingly, when a diameter of a pupil E1 is set to 2 mm, eyes E can see an image even when the eyes are moved in a range of total 8 mm in which each radius of 1 mm of the pupil E1 is respectively added to both sides of the diameter of 6 mm of the light flux L1. That is, since the outermost light beam is incident on the pupil E1 even when the eyes E are positioned at a position PA or a position PC, the eyes can see the image. However, in a configuration illustrated in FIG. 10, since intervals between the −1-st order diffraction light, the 0-th order diffraction light, and the +1-st order diffraction light are wide and may be respectively 3 mm, if the eyes E is positioned at a position PB between the 0-th order diffraction light and the +1-st order diffraction light, there is a problem in that the light flux L1 is not incident on the pupil E1, such that it is not possible for the eyes to see the image.